Single-phase transformers use the ratio between the primary and secondary windings to step up the input voltage. Three-phase transformers work the same way, but they are configured differently. Instead of a primary and a secondary winding, three-phase transformers have primary and secondary cores. Each core consists of three single-phase windings, one winding for each line. Three-phase transformers come in four types: delta to delta, delta to wye, wye to delta, and wye to wye. They differ in the way the primary and secondary cores are interconnected. Engineers use the delta-wye configuration in most commercial applications.
Turn off all power to the system. Put on electrical safety gloves and observe standard electrical safety precautions.
Locate a three-phase transformer with a delta-wye configuration. Make sure that the cores of the transformer have the proper step-up ratio. Find the step-up ratio by dividing the output voltage by the input voltage. For example, if you are looking to step up from 208 volts to 240 volts, you divide 240 by 208 to get 1.15. The step-up ratio is 1.15 to 1, or 1.15:1.
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Position the transformer between the three-phase source and the three-phase load. Locate the three input wires on the three-phase source. Each wire represents one phase.
Connect the three input wires from the source to the three input terminals on the primary, or "delta," side of the transformer. (See Figure 1-2: Delta-Wye connection, in Reference 1.)
Find the main ground of the power source. For most three-phase systems, the main ground is located in the central distribution panel. Connect the neutral of the transformer to the main ground.